1. Field of the Invention
The invention lies in the mechanical arts. More specifically, the invention relates to a method for attaching a micromechanical semiconductor sensor in a housing and a sensor assembly having a micromechanical sensor arranged in a cavity in a housing.
2. Description of the Related Art
European patent EP 0 548 470 B1 discloses a sensor assembly with a micromechanical pressure sensor. The pressure sensor is protected by a corrosion-proof cover formed of several layers. A silicone gel serves as the first layer of the cover and as a pressure transmitting medium.
U.S. Pat. No. 4,866,989 discloses a pressure transducer which has a chip on the underside of a wall of a sensor attachment. The chip is attached to the wall with a sealing ring made of silicone. The chip and the sealing ring are covered by a silicone gel. The silicone gel protects the chip against a fluid whose pressure is being measured.
German published patent application DE 137 03 206 A1 relates to a pressure sensor component with a hose attachment. A semiconductor chip is bonded to a chip carrier of a base body. The chip carrier is filled with silicone gel in order to cover the semiconductor chip. At the same time, by filling in the chip carrier, the base body is connected to a connecting element and sealed.
U.S. Pat. No. 4,655,088 discloses a pressure transducer with a one piece housing and a semiconductor chip which is secured in a defined position by means of a bonding material. The semiconductor chip is additionally protected against environmental influences by a protective material.
European patent EP 0 400 074 B1 discloses a method for fabricating a protected pressure sensor assembly. An interior space around a pressure sensor element is at least partially filled with a pressure transfer material and is closed off by a flexible, pourable diaphragm material.
European patent EP 0 568 781 B1 discloses a pressure sensor assembly with a receptacle which comprises a first recess and a second recess. A semiconductor chip is arranged on a glass substrate in the first recess cavity. Both the first recess and the second recess are provided with gelatinous material. The second recess serves as a working area from which excess gelatinous material can be removed from the first recess.
Micromechanical semiconductor sensors react sensitively to mechanical or thermal stresses in the silicon chip. In the prior art micromechanical sensors, the unhoused sensor chip is bonded to a surface. As a result, interactions with the underlying surface occur. The underlying surface expands differently than the chip, the bonding expands as a result of moisture, the mechanical clamping of the entire housing exerts stresses on the chip and the bonding agent responds to temperature changes by plastic deformation. In addition the plastic housing shrinks after the injection.
In order to reduce the interactions between the sensor chip and its underlying surface, the sensor chip is frequently glued, anodically bonded, or soldered with glass solder onto a ceramic carrier, another silicon component or Pyrex glass. Such a carrier is then bonded into the plastic housing so that two connection points, which are relevant for quality, are produced. Both connection points have effects on the characteristic curve of the sensor. Even minute air bubbles in the bonding agent lead to quality problems.
It is accordingly an object of the invention to provide a method of attaching a micromechanical semiconductor sensor in a housing and a sensor assembly, which overcome the above-mentioned disadvantages of the heretofore-known devices and methods of this general type, and which permit particularly good isolation of a semiconductor sensor from its underlying surface.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of mounting a micromechanical semiconductor sensor in a housing, which comprises the following steps:
positioning a micromechanical sensor on a supporting surface in a recess of a housing;
electrically connecting the micromechanical sensor with an electrical contact on the housing and securing the micromechanical sensor to the housing during the connecting step by generating a partial vacuum and holding the micromechanical sensor with the partial vacuum via a duct in the housing; and
filling gel into the recess for attaching the micromechanical sensor to the housing, so that the micromechanical sensor is covered and held by gel.
In other words, while the electrical connection is produced, the semiconductor sensor is secured by generating a partial vacuum or a flow of air on its supporting surface.
Dispensing with an essentially rigid connection between the semiconductor sensor and its supporting surface brings about an isolation of the sensor from the housing. The semiconductor sensor is essentially secured by means of a gel which simultaneously protects the sensor against environmental influences. A fluorized silicone gel, which has very stable behavior with respect to chemical influences, is particularly suitable for this. The viscous or soft, elastic properties of the fluorized silicone gel prevent any interaction between the semiconductor sensor and the housing and thus prevent the sensor characteristic curve from being influenced. The electrical connection between the semiconductor sensor and an electrical contact on the housing can serve as an additional securing means.
It is possible to dispense with the attachment of a carrier to the semiconductor sensor and with bonding in the housing. As a result the fabrication of a sensor assembly according to the invention is made simpler and cheaper. A special diaphragm for protecting the semiconductor sensor is not necessary either.
In accordance with an added feature of the invention, gel is aspirated via the duct.
In accordance with an additional feature of the invention, a layer of gel is formed between the micromechanical sensor and the supporting surface by aspirating the gel via the duct.
In accordance with another feature of the invention, a suction device is applied to a side of the housing in which the recess is formed.
In accordance with a further feature of the invention, the duct is a groove in the housing and the duct is covered with a suction device during the aspirating process.
In accordance with another added feature of the invention, a sealing lip of a suction device is placed in an area of movement of a bonding device bonding the electrical connections to the micromechanical sensor, and wherein the sealing lip is at least partially countersunk in the housing.
With the above and other objects in view there is also provided, in accordance with the invention, a sensor assembly, comprising:
a housing having a recess formed therein with a supporting surface;
a micromechanical sensor disposed on the supporting surface;
a gel covering the sensor and exclusively securing the sensor; and
the housing having a duct formed therein starting at a side of the recess and extending into the supporting surface such that the micromechanical sensor at least partially covers the duct.
In accordance with another added feature of the invention, the duct is a groove formed in the housing. In a further preferred embodiment, the duct has an opening formed on the same side of the housing as the recess.
In accordance with a concomitant feature of the invention, the recess is formed with lateral walls defining a spacing gap of from 0.05 mm to 1 mm between the micromechanical sensor and the lateral walls.
In the preferred embodiment of the invention, therefore, the sensor assembly not only has the recess but also has a duct which extends to the supporting face of the micromechanical sensor, so that an ambient pressure acts on both planar sides of the micromechanical sensor. As a result, even at a low ambient pressure a pressure differential is prevented from lifting the micromechanical sensor from its underlying surface, in which it is secured solely by the gel or at least essentially by the gel. Damage to the electrical connections to the micromechanical sensor is thus prevented.
Suitable fields of application of the invention are, for example, acceleration sensors, rotational speed sensors and chemical sensors for determining the composition of mixtures of gases. A particularly preferred field of application of the invention is the field micromechanical pressure sensors such as are used, in particular, in automotive engineering. Such pressure sensors are produced, for example, in surface micromechanics or bulk micromechanics.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for attaching a micromechanical sensor in a housing and sensor assembly, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.